In conventional detectors the equilibrium oxygen partial pressure of the atmosphere or of a metal and its metal oxide is utilized as the reference gas, but the following drawbacks are recognized with these detectors. For instance, in a detector which utilizes the atmosphere as the reference oxygen partial pressure, an air duct is required and the infiltration of water, sand or salt etc. into said air duct has to be prevented, which renders the detector complicated in structure. If a mixture of a metal and its metal oxide is used for the source of the reference oxygen partial pressure such as Ni/NiO, Cu/CuO, Co/CoO, Fe/FeO reacts with the vessel of solid electrolyte and accordingly a long continued use of the detector with stability cannot be expected.
In the case of a detector using a solid reference oxygen pole, in which a lead wire is buried in the mixture of a metal and its metal oxide to transmit the electric output, a high internal resistance of the mixture itself or a high contact resistance between said mixture and the lead wire causes deterioration of the low temperature performance when the detector is assembled in the system.